WO2012148997A2

WO2012148997A2 - Thermal energy storage devices, systems and heat storing methods for efficient long term heat storage

Info

Publication number: WO2012148997A2
Application number: PCT/US2012/034962
Authority: WO
Inventors: Andrey N. Soukhojak; Kalyan Sehanobish; David H. Bank
Original assignee: Dow Global Technologies Llc
Priority date: 2011-04-25
Filing date: 2012-04-25
Publication date: 2012-11-01
Also published as: WO2012148997A3

Abstract

The invention is directed at devices 10 and systems 120 for thermal energy storage, and far process, of storing energy using these devices and systems. The devices include a flow path 4 that includes a plurality of sections that are arranged to reduce or minimise heat lessee from the device from heat flow along the flow path. The flow path 4 preferably includes two or more caged flow path sections 20. The device includes :.a thermal energy storage material 2 suitable for storing heat, The devices may be particularly useful for storing heat for Long periods of time.

Description

THERMAL ENERGY STORAGE DEVICES, SYSTEMS AND HEAT STORING METHODS FOR EFFICIENT LONG TERM HEAT STORAGE

CLAIM OF BENEFIT OF FILING DATE

[001] The present application claims the benefit of the filing date of U.S. Provisional Application Serial No. 61/478,723, filed on April 25, 2011 , which is hereby incorporated by reference for all purposes.

[002] FIELD OF THE INVENTION

[003] The present invention relates to thermal energy storage using a thermal energy storage material (i.e., TESM) and to the packaging of the TESM and to components for carrying a heat transfer fluid that allow for efficient heat storage. In particular, the invention relates to storage of heat, such as heat from solar sources, where thermal losses are reduced or minimized.

[004] BACKGROUND OF THE INVENTION

[005] Thermal energy storage materials, such as phase change materials, have been used in applications for storing heat for subsequent use. These materials have been used in devices that store and discharge thermal energy. Examples of such devices include the heat batteries described in U.S. Patent Nos. 7,225,860; 6.784,356; and 6,102,103. Heat batteries have been proposed for use in a number of applications, such as to improve catalytic efficiency (see e.g., U.S. Patent No. 6,875,407), for warming an engine or passenger compartment of a vehicle (see e.g., U.S. Patent Nos. 6,102,103).

[006] Despite efforts to develop such heat storage devices, it is observed that their structures may vary, dependent upon such factors as the desired operating temperatures to which the systems are exposed, the desired rate of heat exchange, the nature of the phase change materials employed or others. Structural variations in heat storage devices include variations in the way the phase change material is packaged, variations in the way the phase change material interacts with a heat transfer fluid, variations in the manner that a packaged phase change material is arranged in a tank or container of the device, and variations in the way the structures contain the phase change materials. For example, U.S. Patent No. 7,225,860 purports to depict the use of encapsulation tubes to hold a phase change material, and U.S. Patent No. 6, 102,103 purports to depict a jacket to contain a phase change materials.

[007] One potential application for a heat storage device is for heating a building using solar heat. Because of the cyclical nature of solar energy's availability (day-night cycles and seasonal cycles) and the unpredictability of solar energy (storms and clouds), there is a problem when the supply of solar energy does not meet the demand for the energy. Two general approaches are possible for storing solar heat. In a first approach, a heat storage device is employed to store solar heat for a few days or a few weeks so that there is energy available for nighttime demand and during inclement days, such as cloudy or stormy days. In this approach, a fairly small heat storage device can be used, but the heat storage system will require a generally large solar collector to capture sufficient amounts of solar energy during winter months. Because these systems require such a large solar collector, these systems may be cost-prohibitive, space-prohibitive or both. Additionally, these systems may be inefficient in that the large solar collector may be underutilized during all but the coldest months. In a second approach, a much larger heat storage device is employed to capture and store solar heat during the warm season and then using the stored heat during the cold season. Here, it may be necessary for a heat storage device to be sufficiently large to hold the heat required for a period of 30 days or longer, or even 90 days or longer. As such, the drawback with this second approach has traditionally been the cost and size requirements of the heat storage device. For example, such a heat storage device needs to meet two competing design requirements— the ability to quickly remove heat for heating a building, or other object, and the need to store heat for a long period of time. For example, devices for storing heat for a long period often require expensive insulation which adds to the cost and size of the device. Such an insulating layer may be particularly thick and expensive in a device that is required to store heat for several months and/or at a temperature that is substantially different than ambient conditions. Some of the heat losses over time may be due to heat flow through the phase change material to an outer surface of a device. Even when a low thermal conductivity phase change material is employed, the heat losses are generally very high due to heat flowing along the length of piping that runs through the device for carrying a heat transfer fluid. For example, when the phase change material has a low thermal conductivity, it may be necessary to use a large amount of piping running through the device so that the required charge and/or discharge power ratings of the device are maintained.

[008] There continues to be a need for cost-effective heat storage devices that are capable of storing heat for extended periods of time, and / or require less insulation, or even no insulation. [009] SUMMARY OF THE INVENTION

[0010] The various aspects of the present invention meet one or more of the aforementioned needs using an approach that reduces, minimizes, or even eliminates the need for insulation of a heat storage device. The materials employed for the various components of the invention, the shape and arrangements of the various structures, or any combination thereof, may be employed to reduce the heat losses.

[0011] One aspect of the invention is directed at a heat storage device comprising: a tank having a cavity; a thermal energy storage material (i.e., TESM) in the cavity of the tank. The tank may include two or more openings so that a heat transfer fluid can enter and exit the cavity of the tank through different openings. The heat storage device includes and one or more flow path structures within the cavity that define a predetermined flow path (e.g., a predetermined fluid flow path) through the cavity of the tank. The flow path structures typically are configured to prevent the thermal energy storage material from directly contacting the heat transfer fluid. The flow path structures are in fluid communication with two or more openings of the tank. The flow path includes two or more caged flow path sections, where each caged flow path section defines a cage (i.e., a boundary surface with a bounded volume inside the cage). The flow path includes a first caged flow path section that defines a cage wherein the cavity has a space inside the boundary of the first caged flow path section and a space outside the boundary of the first caged flow path section. The flow path includes a second caged flow path section that is smaller than the first caged flow path and is located within the space inside the boundary of the first caged flow path section. The flow path also includes a gradient flow path section that provides a fluid connection between the first and second caged flow path sections so that a heat transfer fluid can flow serially through both the first and second flow path sections. Preferably, the flow path includes 3 or more caged flow path sections that are serially connected.

[0012] Another aspect of the invention is directed at a heat storage device comprising: a tank having a cavity, and two or more openings for flowing a heat transfer fluid through the cavity; one or more pipes in the cavity of the tank and in fluid communication with two or more openings so that a heat transfer fluid can flow through the one or more pipes. The one or more pipes include a plurality of isothermal portions of pipe that each have a plurality of windings. The one or more pipes include: a first isothermal portion of pipe having a plurality of windings including adjacent windings having different circumferences, and having a length direction that generally follows a first isothermal contour surface; and a second isothermal portion of pipe having a plurality of windings including adjacent windings having different circumferences, and having a length direction that generally follows a second isothermal contour surface interior to the first isothermal contour surface; so that heat flow along the length of the isothermal portions of pipe is reduced or minimized. The heat storage device includes a thermal energy storage material in the cavity and exterior to the pipe, wherein the thermal energy storage material is in thermal communication with the pipe The one or more pipes preferably have sufficiently high thermal conductivity and surface area so that heat can be transferred between the thermal energy storage material and a heat transfer fluid flowing through the pipe.

[0013] Another aspect of the invention is directed at a system for storing heat including a heat storage device, such as a heat storage device according to the teachings herein, wherein the system includes a heat source; wherein the heat source is heated using solar radiation; and wherein the heat storage device and the heat source are in thermal communication at least during one or more instances when the heat source has a temperature greater than the temperature of some or all of the thermal energy storage material in the heat storage device.

[0014] Another aspect of the invention is directed at the use of a heat storage device, such as a heat storage device according to the teachings herein, for heating a structure requiring heat, such as a building, a water tank, an industrial process, or any combination thereof.

[0015] A method related aspect of the invention is directed at a process comprising: a step of flowing a heat transfer fluid through a flow path in a cavity of a tank. The step of flowing through a flow path includes serially flowing through two or more caged flow path sections, including: serially flowing through i) a first caged flow path section having an adjacent caged flow path section; ii) a gradient flow path section that provides a fluid connection between the two adjacent caged flow path sections; and iii) the adjacent caged flow path section. The two adjacent caged flow path sections generally have different sizes and each define a bounded volume. One of the caged flow paths sections is located within the bounded volume of the other.

[0016] Another method related aspect of the invention is directed at a process comprising: a step of charging a heat storage device, such as a heat storage device according to the teachings herein, including steps of i)flowing a heat transfer fluid into an opening of a tank and through the flow path in the cavity of the tank, wherein the heat transfer fluid has an initial temperature that is greater than a temperature of the phase change material contained in the cavity; ii) contacting the heat transfer fluid to a wall along a caged flow path section; iii) transferring heat from the heat transfer fluid to the wall; iv) conducting the heat through the wall to a phase change material; v) melting at least a portion of the phase change material; and flowing the heat transfer fluid out of the cavity and through a second opening wherein the heat transfer fluid has a temperature lower than its initial temperature.

[0017] BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The present invention is further described in the detailed description which follows, in reference to the noted plurality of drawings by way of non-limiting examples of embodiments of the present invention, in which like reference numerals represent similar parts throughout the several views of the drawings, and wherein:

[0019] FIG. 1A is a schematic drawing showing an illustrative heat storage device having a tank and a view of components that may be within a tank

[0020] FIG. 1B is a schematic drawing showing a solid material that fills the entire space of a tank.

[0021] FIG. 2 is an illustrative graph of isotherms (i.e. surfaces of equal temperature) in a heat storage device, such as a device that is slowly losing stored heat to the environment via external surfaces.

[0022] FIG. 3A is a schematic drawing of an illustrative caged structure. As illustrated in FIG.

3A, a caged structure may include double-walled container.

[0023] FIG. 3B is a cross-section of an illustrative heat storage device including a plurality of caged structures.

[0024] FIG. 4A is a top view of an illustrative caged structure.

[0025] FIG. 4B is an axonometric projection of the caged structure of FIG. 4A. As illustrated in

FIG. 4B, a caged structure may includes a plurality of pipes.

[0026] FIG. 5 is a front view of a pipe wound to form three nested caged structures and gradient flow structures connecting the adjacent caged structures.

[0027] FIG. 6 is a cross-sectional view of FIG. 5. The caged structures may have a generally ellipsoidal arrangement.

[0028] FIG. 7 is an axonometric projection of a pipe wound to form three nested caged structures and gradient flow structures connecting the adjacent caged structures.

[0029] FIG. 8A is a cross-sectional front view of FIG. 7.

[0030] FIG. 8B is a cut-through view of a heat storage device having a generally cylindrical shaped tank with a wedge shaped portion removed to show illustrative features of the components that may be in the cavity of the tank. [0031] FIG. 9 is a front view of an illustrative portion of a wound pipe that includes heat conducting components for improving the heat flow between the pipe and the TESM.

[0032] FIG. 10 is a cross-sectional view of an illustrative heat conducting component.

[0033] FIG. 1 1 is a cross-sectional view of an illustrative heat storage device having a generally ellipsoidal shape and having ellipsoidal shaped caged structures. As illustrated by FIG. 11 , a caged structure may divided into two halves, such as two half ellipsoids.

[0034] FIG. 12 is a schematic drawing of an illustrative heat storage system. A heat storage system may include one or any combination of the elements shown in this figure.

[0035] DETAILED DESCRIPTION

[0036] The explanations and illustrations presented herein are intended to acquaint others skilled in the art with the invention, its principles, and its practical application. Those skilled in the art may adapt and apply the invention in its numerous forms, as may be best suited to the requirements of a particular use. Accordingly, the specific embodiments of the present invention as set forth are not intended as being exhaustive or limiting of the teachings. The scope of the teachings should, therefore, be determined not with reference to the above description, but should instead be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. The disclosures of all articles and references, including patent applications and publications, are incorporated by reference for all purposes. Other combinations are also possible as will be gleaned from the following claims, which are also hereby incorporated by reference into this written description. In the following detailed description, the specific embodiments of the present invention are described in connection with its preferred embodiments. However, to the extent that the following description is specific to a particular embodiment or a particular use of the present techniques, it is intended to be illustrative only and merely provides a concise description of the exemplary embodiments. Accordingly, the invention is not limited to the specific embodiments described below, but rather; the invention includes all alternatives, modifications, and equivalents falling within the true scope of the appended claims. The present invention is further illustrated by the attached figures.

[0037] The heat storage devices according the teachings herein may reduce, minimize or eliminate the need for insulation, may increase or maximize the volume fraction of the device that is occupied by TESM, or both. The novel designs of the heat storage devices may further permit a high power rating of the device (e.g., a power rating above a minimum required level). The rate of heat transfer may be the rate of heat transferred between a heat transfer fluid (i.e., HTF) and the TESM.

[0038] Some of the features of a heat storage device are illustrated in FIG. 1A. The heat storage device 10 includes a tank 6 having a cavity 7. The heat storage device 10 typically includes one or more thermal energy storage material 2 in the cavity 7 of the tank 6. The tank 6 should be capable of containing the TESM 2 in both a solid state and in a liquid state. The heat storage device 10 may include a flow path 4 suitable for flowing a heat transfer fluid 8 through multiple regions within the cavity 7 of the tank 6. Preferably the flow path 4 is defined by one or more structures, such as a pipe wall 12, which prevents the HTF 8 from directly contacting the TESM 2. The device preferably includes two or more openings 14 so that the HTF can enter the flow path 4 through one opening 14 and exit the flow path 4 through a different opening 14. It will be appreciated according to the teachings herein that flow path 4 may be defined by structures other than pipes. For example, a flow path may be defined by the space between opposing walls of a double-wall container.

[0039] Flow paths

[0040] The device includes a flow path suitable for a HTF entering the cavity of a tank through an opening of the tank, flowing through various regions within the cavity of the tank, and exiting the tank by flowing through a different opening of the tank. The cavity of the tank is primarily filled with a TESM, and the flow of the path may prevent the HTF from directly contacting the TESM.

[0041] The flow path include a caged flow path sections that cage or otherwise enclose a space (i.e., a bounded volume). The flow path generally includes 2 or more, preferably 3 or more, and more preferably 4 or more caged flow path sections. Each caged flow path section defines a 2-dimensional boundary surface that divides the space within the cage and the space outside of the cage. In other words, the 2-dimensional boundary surface definded by the caged flow path section surrounds a space. A caged flow path section preferably has a structure that forms a top boundary, a bottom boundary, and side boundaries. For example, there bounded volume may be bounded in positive and negative directions in each of the x, y, z, Cartesian coordinates. The caged flow path section may define a boundary that distinguishes a space in the tank that is above the cage from a space within the cage, that distinguishes a space in the tank that is below the cage from a space within the cage, that distinguishes a space within the tank that is exterior to a side (e.g., front, rear, right, left, or any combination thereof) of the cage from a space within the cage, or any combination thereof. The caged flow path sections preferably have different sizes. As such, the flow path may include a caged flow path section that is an innermost caged flow path section which is located inside the space caged by an outermost flow path section. The flow path may include one or more intermediate caged flow path sections which are located inside the outermost caged flow path section and outside of the innermost caged flow path section. Preferably the special relationship between each pair of caged flow path sections may be described as a first caged flow path section located entirely within the space of a larger caged flow path section. By way of illustration, the caged flow path sections may be nested like so many Russian dolls.

[0042] A function of caged flow path section may be to reduce or minimize heat loss due to heat flow along the length of the flow path when HTF is not flowing. As such, a caged flow path section may be configured to follow a generally isothermal contour, so that the variation in temperature of the TESM adjacent to different regions of a caged flow path section is reduced or minimized.

[0043] The flow path also includes one or more gradient flow path sections. One or more gradient flow path sections may provide a fluid connection between two caged flow path sections. A gradient flow path section may provide a fluid connection between a caged flow path and an opening in a tank. A pair of adjacent caged flow path sections may be connected by one or more gradient flow path sections so that a HTF can flow serially (e.g., sequentially) through the two gradient flow path sections. Preferably a pair of adjacent caged flow path sections are connected by a single gradient flow path section. Preferably, each pair of adjacent caged flow path sections is connected by one or more gradient flow path sections so that a HTF can flow serially through all of the caged flow path sections. A gradient flow path section may provide a fluid connection between an innermost caged flow path section and an opening of the tank. A gradient flow path section may provide a fluid connection between an outermost caged flow path section and an opening of the tank. A gradient flow path section may provide a fluid connection between an opening of the tank and a caged flow path section located between the innermost and outermost caged flow path sections. The gradient flow path sections may be arranged so that a HTF flows sequentially through 2 or more, 3 or more, or even every caged flow path section. A gradient flow path section may provide a fluid connection between an opening of the tank and a caged flow path section located between the innermost and outermost caged flow path sections. Such a gradient flow path section may be employed so that a HTF does not flow through both the innermost and the outermost caged flow path sections.

[0044] While in a gradient flow path, the HTF preferably flow in a direction that is generally towards the center of the cavity of the housing or generally away from the center of the cavity of the housing. For example, the acute angle between the direction of flow in the gradient flow path and the vector to or from the center of the center of the cavity may be about 40 ° or less, about 30 " or less, about 20 ^* or (ess, or about 10 ° or less.

[0045] While operating a heat storage device in a heat storing mode, a gradient flow path section will generally traverse past TESM having different temperatures. The gradient flow path may include one or more of the following features to decrease or minimize heat loss along the length of the gradient flow path section: the gradient flow path may be generally straight, the length of the gradient flow path may be about the distance between the components it connects, the gradient flow path section may be insulated, the gradient flow path may have a generally small cross-section, or the gradient flow path may be defined by walls formed of material having a generally low thermal conductivity.

[0046] The flow of a HTF through the device may include one or more, or even all of the following steps: flowing the HTF through a first opening of the tank; flowing the HTF through a gradient flow path section that provides a fluid connection between the opening and an innermost caged flow path section; flowing the HTF through the innermost caged flow path section; flowing the HTF through a gradient flow path sections that provides a fluid connection between the innermost caged flow path section and an adjacent intermediate flow path section; flowing the HTF through an intermediate flow path section flowing the HTF through a gradient flow path sections that provides a fluid connection between the an intermediate flow path section and an adjacent outermost caged flow path section; flowing the HTF through the outermost caged flow section; flowing the HTF through a gradient flow path section that provides a fluid connection between the outermost caged flow path section and a second opening of the tank; or flowing a HTF through the second opening of the tank. One or more of the aforementioned steps may be done in sequential order. One or more of the aforementioned steps may be done in reverse sequential order. The flow of a HTF through the device may be for charging the device by transferring heat from the HTF into the device. The flow of HTF through the device may be for discharging the device by transferring heat from the device to the HTF. Preferably during the charging, the HTF flows sequentially through continuously larger caged flow path sections. For example, the HTF may flow sequentially in a serial manner from the innermost caged flow path section, then through each adjacent intermediate caged flow path section, and then through the outermost caged flow path section. Preferably during discharging of the device, the HTF flows sequentially through smaller caged flow path sections. For example, the HTF may flow sequentially in a serial manner from the outermost caged flow path section, then through each adjacent intermediate caged flow path section, and then through the innermost caged flow path section.

[0047] By flowing the HTF sequentially through a series of caged flow path sections, it may be possible to maintain the TESM near the center of the device at a generally high temperature, for longer times than traditional flow paths used in charging and discharging a heat storage device. For example, the TESM may be maintained at a temperature above its melting temperature for longer times than a device that includes a similar length of flow paths that continuously crosses through temperature isotherms, such as taught by Shin in U.S. Patent No. 3,163,209 (see e.g., column 1 , lines 26, to column 3, line 9, and figures 1 and 3, incorporated herein by reference). Shin teaches a plurality of spiral shaped flow paths that are generally concentric. However, each of Shin's spiral flow paths continues to the surface of the tank and does not define a boundary of space within the tank that is above the spiral or below the spiral.

[0048] A caged flow path section may follow a more isothermal contour within the cavity of the tank, which is facilitated by the caged structure having a top and bottom boundary as well as side boundaries. Preferably, the caged flow path section is arranged and positioned within the cavity of the tank so that the flow through the caged flow path generally follows an isothermal contour. As such, a caged flow path section may be a generally isothermal flow path section.

[0049] An isothermal flow path may allow a fluid to flow generally along a single isotherm.

When no fluid is flowing through the isothermal flow path, the TESM along the isothermal flow path will have substantially uniform temperate. Using such a flow path, the vector of the gradient of the temperature typically will be generally perpendicular to the direction of flow at substantially all, or even entirely all locations within the isothermal flow path. As used herein, generally perpendicular means within 25° of the perpendicular. The angle between the direction of flow and the vector of the temperature gradient is preferably about 70" or more, more preferably about 80° or more, even more preferably about 86° or more, even more preferably about 88° or more, and most preferably about 89° or more. The angle between the direction of flow and the vector of the temperature gradient may be even be 90°

[0050] Such a configuration may result in reduced or even minimal heat flow along the length of the isothermal flow path when fluid is not flowing. In contrast, flow paths used in the prior art heat storage devices (see e.g., FIG. 1 of US Patent No. 3,163,209), will allow heat to flow from the regions of a pipe near the center of a cavity of the tank to regions of a pipe near the walls of the tank when the device is in a storing mode, without fluid flowing through the pipe.

[0051] The isotherms in a heat storage device will typically be defined by the shape of the tank holding the TESM and the boundary conditions. The boundary conditions may include the thermal properties (e.g., heat capacity, heat conductivity, thermal convection, thickness) of the tank, including any insulation, and the materials or air that contact the outside of the tank.

[0052] An isothermal contour may be calculated using modeling or other mathematical methods that are generally well known. With reference to FIG. 1 B, consider an illustrative device 10 having a tank 6 that is generally cubic and entirely filled with a solid material 5 having uniform heat capacity and uniform thermal conductivity. The material 5 in the tank 6 may be heated to a generally uniform peak temperature (T_p). The tank is then exposed on all sides to a uniform ambient temperature (T_a) having a lower temperature (T_a < T_p) than the heated material. As heat is lost through the walls of the tank, a temperature gradient is created with the highest temperature near the center (T_c) of the tank and the lowest temperatures near the walls of the tank. When T_c has dropped by about 20% (i.e., T_c = (0.2T_a + 0.8T_P), the temperature profile of a cross-section through the center of the tank may have one or more of the features, such as shown in FIG. 2. In FIG. 2, a plurality of isotherms 24 (24A, 24B, 24C, 24D, 24E, 24F, 24G) are shown. The isotherms with the lowest temperature may be near the wall of the tank. The isotherms with the highest temperature may be near the center of the tank. The isotherms near the wall of the tank may have a shape that is generally the same as the shape of the tank. The vector of the temperature gradient 26 is shown at different locations in the tank. While the device is storing heat with heat losses along the walls of the tank, the center 25 of the tank may have the highest temperature. The isotherms near the center of the tank may have a generally ellipsoidal shape, such as a generally spherical shape (i.e., the cross-section will be generally elliptical or generally circular). Isotherms having a high temperature may be enclosed within one or more, or even all of the isotherms having a lower temperature. In the direction from a wall of the tank to the center of the tank, the magnitude of the temperature gradient may be uniform or may be lower near the center of the cavity of the tank than near one or more walls of the tank. Although FIG. 2 illustrates an isotherm in a 2-dimensional construction as a one-dimensional line, it will be appreciated that an isotherm in the cavity of the tank is typically a 2- dimensional surface.

[0053] The direction of the temperature gradient at a point on an isotherm is the direction normal to the isotherm at that point. For many regular shaped tanks, the direction of the temperature gradient may be approximated by the direction from the geometric center of the cavity of the tank to the point on the isotherm. Such an approximation is particularly accurate at a distance of about 10% or more from a wall of the tank towards the center of the tank.

[0054] It will be appreciated that an isothermal flow path will necessarily have walls that contact TES at different temperatures due to the thickness of the flow path, such as the diameter of the pipe conveying the HTF. The difference between the maximum temperature and the minimum temperature of TESM contacting the walls of a isothermal flow path is the contact temperature range, T_c. It is desirable that within each individual isothermal flow path the contact temperature range be generally low so that heat losses due to heat flow along the length of these walls during a heat storing mode is reduced or minimized. The ratio of the contact temperature range, T_c to the temperature range in the device T_r = (0.8T_pT_a) after the temperature in the center of the drops by 20%, preferably is about 0.3 or less, more preferably about 0.2 or less, even more preferably about 0.15 or less, even more preferably about 0.10 or less, even more preferably about 0.05 or less, and most preferably about 0.02 or less.

[0055] As the caged flow path section may be arranged to generally follow along an isothermal surface, it will be appreciated that one or any of the aforementioned features of an isotherm may be used to describe a caged flow path section, such as an isothermal flow path section. For example, a caged flow path section may be entirely enclosed or bounded within a second caged flow path section.

[0056] The isothermal flow paths should provide sufficient surface area for transferring heat between the HTF and the TESM. The surface area requirements may depend upon design requirements, such as the power input/output required from the device, the temperature requirements, the temperature limitations, and the size of the device.

[0057] It will be appreciated that the various sections of the flow path should be spaced throughout the cavity of the tank so that heat from TESM in the various regions within the cavity can be more easily heated and cooled by the HTF.

[0058] Each flow path section may be defined by one or more of the following spaces: the space in a portion of pipe, the space between two or more walls, or both. A flow path may include the cavity in one or more pipes or portion of a pipe, one or more spaces between two sheets, a space between two containers, or any combination thereof. For example, a flow path section or a portion of a flow path section may be defined by the space between the walls of a double-wall container. Such a double-wall container may have any suitable shape according to the teachings herein that reduces or minimizes the heat loss from heat flowing along one or more walls of the double-walled container in a direction of the flow of the HTF. For example, a double-walled container may include one or both walls having a generally ellipsoidal shape (such as a spherical shape), a generally cuboid shape, a generally cylindrical shape, or any combination thereof. A double-walled container may include a first walled surface that generally nests in a second walled surface. Preferred double-walled containers may be generally geometrically similar in shape, be concentric, or both. For example, a double-walled container may have a generally uniform spacing distance between the walls, such as a ratio of the standard deviation of the spacing distance to the mean spacing distance that is about 70% or less, about 40% or less, about 25% or less, or about 10% or less.

[0059] The caged structures, such as the one or more pipes or double-wall containers that defines the caged flow path sections (e.g., an isothermal flow path) preferably include or consist of materials having generally high thermal conductivity so that the TESM that contacts the outside of the structure is in thermal communications with the HTF between the walls of the structure. Such materials typically have a thermal conductivity greater than the thermal conductivity of the TESM, greater than the thermal conductivity of the material of the structures that forms the gradient flow path sections, or both. The caged structure preferably includes a material having a thermal conductivity greater than for the materials used for a wall of a gradient flow path section. However, according to the teachings herein, the caged structure and the gradient flow path structure may be the same material. For example, according to the teachings herein, a single wound pipe may be used for both a caged structure and a gradient flow path structure.

[0060] The ratio of the thermal conductivity of a material used for the caged structure to the thermal conductivity of the phase change in the solid state, in the liquid state, or preferably both, may be about 1 or more, about 5 or more about 20 or more, about 100 or more, or about 1000 or more.

[0061] The caged structure preferably includes or consists essentially of a metal, a metal alloy, or other material having a thermal conductivity of about 5 Wm^" K^"1 or more, about 10 Wm^"1K^_1 or more, about 15 Wm^" K^"1 or more, about 30 Wm^"1K^"1 or more, or about 200 Wm^' ¹K^"1or more. Such material will typically have a thermal conductivity of about 450 Wm 'K^'1 or less.

[0062] As discussed hereinbefore, the caged structure that defines the caged flow path section (e.g., the isothermal flow path) may be any structure that allows the HTF to flow in a space which confines the liquid along a three-dimensional region having generally the same temperature. By way of illustration, consider a heat storage device having a generally cylindrical shaped tank. A caged flow path section in such a tank may have a generally isothermal flow path that is generally cylindrical in shape. Such a caged structure may be particularly useful towards the walls of the tank. As such, a caged flow path section may be generally geometrically similar, but smaller than the shape of the tank. FIGs. 3A, 4B, and 8 illustrate examples of generally cylindrical flow path sections. FIG. 3A illustrates a flow path that is defined by the space between the walls of a double-walled containers. As shown in FIG. 3A, the hollow cylindrical containers including a smaller cylinder nested within a larger cylinder. The cylindrical containers may have tubular side walls and opposing top and bottom walls, such as illustrated in FIG. 3A. The double-walled containers may have spaced apart walls that allow for the flow of a HTF. The double-walled containers may include one or more openings for allowing the HTF to enter, flow between the walls, and then exit the flow path section. It will be appreciated that other shaped flow paths may similar be formed. For example, a cubic flow path may be formed using spaced apart hollow cubic containers and a spherical flow path may be formed using spaced apart hollow spherical containers.

[0063] FIGs. 4A and 4B illustrates another cylindrical shaped isothermal flow path. The top view is shown in FIG. 4A and the perspective view is shown in FIG. 4B. As illustrated in FIG. 4B, a caged flow path section (e.g., an isothermal flow path) 20 may include one or more pipes 12 or pipe section running along each base 50 of the cylinder and one or more pipes or pipe sections 12 running along the length 52 of the cylindrical shaped cage. The caged flow path section 20 may have two or more openings 18 so that a HTF 8 can flow into and out of the flow path 20. If a plurality of pipes 12 are employed, the pipes preferably will be connected in series, connected in parallel, or both so that a HTF 8 can flow into the flow path 20 at a single opening 18 and out of the flow path at a single opening 18^*. Here, the term connected in parallel is used not to describe the geometrical arrangement, but rather to describe the splitting of the flow into different paths so that a portion flows through one path and a portion flows through another, and fluid does not flow through both paths. With reference to FIG. 4A, the caged flow path section 20 may include an opening 18 into a hub 40 near the center of a first base 36 of the cylinder shape. The hub 40 may divide the flow into a plurality of radial pipes or pipe sections 32 that project radially to the periphery of the base of the cylinder, so that "parallel" flow paths are defined. The lengths of the radial pipes may be in the generally horizontal plane. Each radial pipe may be connected to one or more vertical pipes or pipe sections 34 that extends between the two opposing bases of the cylinder. The base 38 opposing the first base may include a plurality of radial pipes or pipe sections 32 that connect the vertical pipes to a second hub 40. Although Figure 4B illustrates, pipes that are curved and include sections that allow for the radial flow and a section that allows for the vertical flow, such pipes may be replaced with a plurality of pipes that are connected by built-in or separate coupling components. The hubs 40 may be located in any position, but preferably are located centrally so that the flow lengths are about the same. The openings preferably are faced in the same direction so that one opening can easily be connected to another flow path section outside of the caged space and the other opening can easily be connected to another flow path section inside the caged space. It will be appreciated that a cylindrical flow path does not need to cover the entire cylindrical surface, such as in FIG. 3A, but may instead be formed of a structure, such as shown in FIG. 4B including one or more pipes 12 that cover a representative portion of the shape, so that the a cylindrical caged space is defined.

[0064] FIGs. 7, 8A, and 8B illustrate caged flow path section having a generally cylindrical shape. For example, one or both bases of the cylinder may include or even consist of a pipe that spirals between the center of the base and the outer periphery of the base. The caged flow path section may include or even consist of a pipe that spirals between two bases. By way of example, the caged flow path may include pipes or pipe sections that are connected in series so that the HTF is not divided between different pipes. As illustrated in FIGs.7 and 8A, a single wound pipe may be used for the entire caged flow path section.

[0065] A single wound pipe may even be employed for i) two adjacent caged flow path sections and a gradient flow path section coupling the two caged flow path sections, or 2) even an entire flow path within the tank.

[0066] When using a coiled pipe, the pipe may have portions that primarily spiral outwards so that two adjacent windings are nearly on the same plane, the pipe may have portions that primarily spiral outwards so that two adjacent windings have nearly the same circumference, or both. Some or even all pairs of adjacent windings will be both nonplanar and have different circumferences. The distance between adjacent windings is the pitch. A high pitch will generally result in a shorter length of pipe. This may be advantageous so that a large amount of TESM may be stored in the cavity of the tank. However, when the pitch is high, the total surface area of coil may be low, which would limit the rate at which heat can be transferred into and out of the device.

[0067] In order to increase the power input and/or the power output of the heat storage device, the device may include a heat conducting component that provides additional thermal communication between the outer walls of the pipe and the TESM in the cavity of the tank. The heat conducting component may be heat transfer surface extenders that function to extend the surface of the pipe. A heat conducting component may be particularly advantageous when using wound pipe having a large pitch. Preferred heat conducting component have a long direction and a short direction. The short direction of a heat conducting component preferably is arranged within 15 ° or within 10 ^β of the direction from the center of the tank to the heat conducting component. The long direction of a heat conducting component preferably is arranged within 15 ° or within 10 ° of the plane normal to the direction from the center of the tank to the heat conducting component. With such an arrangement, a heat conducting component may have a large surface area for contacting TESM having about the same temperature. For example, the heat conducting component may be shaped and arranged so that it generally fits within a single isotherm. Examples of heat conducting components include a sheet or foil, a fin (such as a serrated fin), a wire rod, and an elongated washer. Preferred heat conducting components have a high aspect ratio (i.e., the ratio of the length to the thickness). The heat conducting component preferably has an aspect ratio of about 1.5 or more, about 2 or more, about 3 or more, or about 5 or more. The heat conducting component may span a portion or all of the distance between two adjacent windings of a pipe. For example, a heat conducting component may span about 10% or more, about 20% or more, or about 30% or more of the distance between two adjacent windings of pipe. A heat conducting component may be provided as a separate component or may be integrated into a pipe. A heat conducting component preferably is in direct contact with a pipe. For example, a heat conducting component may be attached to a pipe. The heat conducting component typically has a high thermal conductivity. For example, the heat conducting component may be made of a material suitable for the caged structure, as discussed herein.

[0068] Total flow lengths of all of the gradient flow path sections is generally short relative to the total flow lengths of the all of the caged flow path sections. For example, the ratio of the total flow length of the gradient flow path to the total flow lengths of all the caged flow path sections may be about 0.3 or less, about 0.2 or less, about 0.1 or less, about 0.03 or less, or about 0.01 or less. The outer walls of the caged structure and the outer walls of the gradient flow structures may both contact TESM. Preferably the majority of the TESM that contacts a flow path structure contacts a wall of the caged structure. For example the ratio of the surface area of the caged structure that contacts TESM to the surface area of the gradient flow structure that contacts TESM may be about 1 or more, about 2 or more, about 4 or more, about 7 or more, or about 10 or more. The gradient flow structure may include one or more thermal barriers that prevent lengthwise heat flow along the entire length of the gradient flow path. For example, two pipes may be connected by a coupling having a relatively low thermal conductivity so that heat flow between the two pipes is reduced or minimized. A connector having low thermal conductivity may also be used to connect a gradient flow structure to a caged structure, where the connecter preferably has a thermal conductivity that is lower than the thermal conductivity of the material of the caged structure.

[0069] According to the teachings herein, the device will includes one or more TESMs located inside the cavity of the tank. The thermal energy storage material may be a phase change material (i.e., PCM) having a liquidus temperature, T_L, at which the PCM begins to undergo a solid to liquid phase transition upon being heated. At T_L, some or all of the PCM undergoes a solid to liquid phase transition. The PCM may be a single PCM or a mixture of PCMs. The PCM may undergo a solid to liquid phase transition over a range of temperatures. Preferably, a major portion of the PCM melts in a generally narrow temperature range, such as a temperature range of about 30 °C or less. For example, about 80 volume percent or more of the PCM may melts in a narrow temperature range of about 20 °C or less, about 10 "C or less, or about 5 °C or less, so that large amounts of heat can be released from the device by heating a HTF to a generally high discharge temperature, T_D. The difference in the discharge temperature of the HTF and the liquidus temperature of the PCM (T_L - T_D) may be about 30 °C or less, about 20 °C or less, about 10 °C or less, or about 5 °C or less, or about 1 ^aC or less. It will be appreciated that the PCM may store heat as latent heat and or sensible heat at temperatures greater than T_L so that T_D is greater than T_L during initial discharge of a heat from the device (i.e., T_L-T_D may be about 0 °C or less).

[0070] It may be particularly desirable for the TESM to have a thermal conductivity sufficiently low so that the TESM is generally self insulating. Such materials may reduce and/or eliminate the need for insulation, particularly if employed with a flow path for the HTF according to the teachings herein.

[0071] During a charging mode of operation, the TESM may receive heat in a step characterized by increasing the temperature of the TESM, increasing the amount of the TESM that is in a liquid state, or both.

[0072] During a maintaining mode of operation (i.e., storing heat for a period of time), the TESM heat preferably loses no heat or only loses heat slowly, such as by heat losses to the environment. During a maintaining mode of operation, a temperature profile may be established with the high temperature near the center of the device and a low temperature near a wall of the tank holding the TESM. If the temperature at a wall, such as a tank wall, is sufficiently reduced over time, the TESM may undergo a liquid to solid phase transition and form a layer of solid PCM on the wall surface. Such a solid PCM preferably has a thermal conductivity that is less than the thermal conductivity of the liquid PCM so that the solid layer acts as a barrier to further heat loss. As the thickness of any solid PCM layer on a wall increases, the rate of heat loss to that wall will continue to decreases. In contrast, during a maintaining mode of operation in the prior art, the walls of the pipes (e.g., of a plurality of flow paths through the tank) allowed heat to be carried to the walls of the tank so that a self- insulating PCM would be ineffective.

[0073] During a discharging mode of operation, the TESM may transfer heat to a HTF in a process characterized by: decreasing the temperature of the TESM, decreasing the amount of the TESM that is in a liquid state, or both.

[0074] During the charging and discharging modes of operation, and after operating in one of these modes, an initial temperature profile may be established with the TESM near the walls of a caged flow path section having a different temperature relative to the TESM more distant from any wall of a caged flow path section. In order to store heat throughout the TESM, it is necessary for the heat to be conducted within the TESM. As such, the TESM preferably has a thermal conductivity sufficiently high so that any TESM that is equidistant from two adjacent caged flow path sections can be heated.

[0075] The TESM may have a generally low thermal conductivity. For example the TESM, in the liquid state, the solid state, or both, may have a thermal conductivity of about 5 Wm^"1K^-1 or less, about 3 Wm^"1K^"1 or less, about 1 Wm^"1K^"1 or less, about 0.8 Wm^"1K^"1 or less, or about 0.5 Wm^"1K^"1or less. Such material will preferably have a thermal conductivity of about 0.008 Wm^{"1 '}' or more. Preferred TESMs have a thermal conductivity that is lower in the solid state than in the liquid state. For example, the ratio of the solid state thermal conductivity to the liquid state thermal conductivity of the TESM may be about 0.95 or less, about 0.8 or less, about 0.7 or less, about 0.6 or less, about 0.5 or less, or about 0.4 or less.

[0076] The amount of heat loss to the environment may depend on the total surface area of tank of the heat storage device. As such, it may be desirable for the tank to be as small as possible, provided that it has the required energy storage capabilities of one or more design requirements. Reduced or minimum tank size may be achieved by i) selecting a TESM having high heat storage capacity, ii) filling a large portion of the cavity of the tank with the TESM, or both. A high concentration of TESM in the cavity of the tank may be achieved selecting caged structures that have a low thickness (i.e., t), by spacing adjacent caged structure a high distance, d, or both. For example, the ratio of t d may be about 0.3 or less, about 0.2 or less, about 0.1 or less, about 0.04 or less, about 0.02 or less, or about 0.01 or less. The spacing between adjacent cages structures preferably is sufficiently small so that three or more, or even four or more caged structures can be arranged in the cavity of the tank, according to the teachings herein. The thickness of the caged structures preferably is sufficiently high so that a HTF can flow through the walled space within the thickness. As such, the ratio of t d is typically about 0.001 or more; however lower values may be used within the teachings herein.

[0077] The number of caged structures in the cavity should be sufficiently high so that the required power requirements (such as power input requirements, power output requirements, or both) of the device is achieved. In addition to the power requirement, the number of caged structures may depend on other factors, such as the temperature difference between the HTF and the TESM during charging, discharging, or both (a low temperature difference may translate into a need for a large number of caged structures), the thermal conductivity of the TESM (a low thermal conductivity may translate into a need for a large number of caged structures).

[0078] The TESM may be any material that undergoes a solid to liquid phase transition, has sufficient heat storage capabilities, and is compatible with the materials in which it will contact.

[0079] Examples of TESMs that may be employed in the heat storage device include the materials described in Atul Sharma, V.V. Tyagi, C.R. Chen, D. Buddhi, "Review on thermal energy storage with TESMs and applications ", Renewable and Sustainable Energy Reviews 13 (2009) 318-345, and in Belen Zalba, Jose Ma Marin, Luisa F. Cabeza, Harald Mehling, "Review on thermal energy storage with phase change: materials, heat transfer analysis and applications", Applied Thermal Engineering 23 (2003) 251-283, both incorporated herein by reference in their entirety. Other examples of suitable TESMs that may be employed in the heat transfer device include the TESMs described in U.S. Patent Application No. 12/389,416 entitled "Thermal Energy Storage Materials" and filed on February 20, 2009; and U.S. Patent Application No. 12/389,598 entitled "Heat Storage Devices" and filed on February 20, 2009, and PCT Application No. PCT/US09/67823 entitled "Heat Transfer Systems Utilizing Thermal Energy Storage Materials," filed on December 14, 2009, ail incorporated herein by reference.

[0080] The tank for the heat storage device may have any shape capable of holding a TESM in a cavity. The shape of the tank may have a regular shape or may have an irregular shape so that it can fit into an irregular space. The tank preferably has a regular shape. For example, the tank may have a shape that is generally spherical, cylindrical, barrel-like, cubic, or prism shaped. The tank may have corners and/or edges that meet at an angle or that are rounded. Preferred shapes are those having generally low surface area, generally high volume (e.g., so that the cavity can have a high volume), or both. The ratio of the volume of the cavity to the total surface area of the outer walls of the tank preferably is generally high. For example the ratio of the volume of the cavity to the total surface area of the outer walls of the tank may be about D/20 or more, about D/15 or more, about D/10 or more, or about D/8 or more, where D is the largest dimension of the tank.

[0081] The tank has a large cavity for holding the TESM. The walls of the tank should be sufficiently thick and / or sufficiently strong so that the tank can contain the TESM without leaking. The tank will typically have openings or other means for pumping a HTF through the cavity. Preferably, the tank includes two openings for flowing HTF through the cavity. The walls of the tank should be sufficiently thin so that the tank holds a large volume of TESM. The walls of the tank may be formed of one or more materials. Preferably, the walls of the tank do not degrade upon contact with the TESM, e.g., when the TESM is in a liquid state. As such, the selection of the materials for the tank may depend on the TESM to be employed, the temperature operating range of the device, or both. The tank may be made of a ceramic material, a polymeric material, a metallic material, a composite material, a concrete material, or any combination thereof. For example, a tank may have multiple layers including a first layer of a material that provides mechanical performance and a second layer that protects the first layer from contacting the TESM.

[0082] The heat storage device may include one or more support components suitable for supporting a caged structure. For example, a support component may be used for positioning the windings of a pipe. A support component may include a spacer for maintaining a desired separation between two caged structures. It will be appreciated that a support component may pass through different temperature isotherms. As such, it is preferable that any support component be made of a material having a generally low thermal conductivity. For example, the thermal conductivity of a support component may be less than the thermal conductivity of the material of the caged structure, less than the thermal conductivity of the gradient flow structure, less than the thermal conductivity of the TESM, or any combination thereof. Preferred support components for the caged structure are sufficiently rigid so that the caged structure is supported even when the TESM is heated above its liquidus temperature.

[0083] FIG. 9 illustrates a portion of a caged structure 29 that includes a plurality of windings of a pipe 42. The spacing between windings and the circumference of each winding may be critical for reducing heat losses. A predetermined arrangement of the pipe 42 may be maintained by one or more support components 44. It will be appreciated that a single support component 44 may support and/or position a plurality of windings of pipe 29, a single winding of pipe 42 may be supported and/or positioned by a plurality of support components 44, or both. The pipe 42 may also have one or more surface extender components 60. As illustrated in FIG. 9, a surface extender component 60 may be held in place by the pipe 42. Along different windings of the pipe 42 and/or along different regions of a single winding, the surface extenders components 60 may have different shapes. For example the elongated portion 64 of the surface extender component 60 may be oriented so that the surface extender component remains generally along the surface of a single isotherm. The surface extender component 60 may have a portion 62 that is in direct contact with the pipe 42 so that good thermal contact is achieved. FIG. 10 is a cross-sectional view of a pipe 42 having a surface extender component that includes a plurality of wires that extend from the outside surface of the pipe 42 along a generally isothermal direction. FIG. 1 1 is a cross-section of a heat storage device having a tank 6 that is ellipsoidal. The device includes a flow path 4 having three caged flow path sections 20, including an innermost caged flow path section 47 an outermost caged flow path section 48 and an intermediated caged flow path section. Each pair of adjacent caged flow path sections 20 may be connected with a gradient flow path section 30. The gradient flow path sections 30 may be formed by pipes having low thermal conductivity compared with the pipes forming a caged flow path section. When using an ellipsoidal tank 6, all of the caged flow path sections 20 may have generally ellipsoidal shape.

[0084] The heat storage device may include one or more barrier components that reduce or prevent convective flow of the TESM within the cavity of the tank. By reducing the convective flow, it may be possible to maintain stored heat in the device for even longer times. It may be advantages for the barrier component to be made of a material having low thermal conductivity so that heat losses from the device are reduced or minimized. It will be appreciated that a caged structure that includes a double-walled container may be sufficiently solid that convective flow of the TESM between the inside of the cage and the outside of the cage is prevented. However, in order to fill a device with TESM, it may be desirable to have a sufficient number of passageways through the double-walled container to allow easy filling of the entire tank with TESM. Of course, any such passageways should still prevent contact between TESM and the HTF.

[0085] Because of the reductions in heat loss when using the caged structures according to the teachings herein, the need for insulation may be reduced, minimized, or eliminated. If insulation is still needed for an application, any art known insulation may be used. Suitable insulating materials will generally have a thermal conductivity (e.g., an effective thermal conductivity) that is less than the thermal conductivity of the TESM. For example, the ratio of the thermal conductivity of an insulating material to the thermal conductivity of the TESM may be about 0.8 or less, about 0.1 or less, about 0.01 or less or about 0.001 or less.

[0086] The heat storage devices and/or caged flow path sections according to the teachings herein may be used in heat storage systems that receive heat from one component, stores the heat for a period of time, and then transfer the heat to the same component or to a different component. The heat storage system may include one or any combination of the following components: a heat source, a heat storage device, one or more loops for circulating a HTF, one or more heat exchanger, one or more sensors, one or more controllers, or one or more HTFs. Particularly desirable heat storage systems are those that require storage of heat for generally long periods of time, storage of large quantities of heat, or both. For example, the heat storage system may store heat from an energy source for later use when the energy source is unavailable, the energy source has reduced power, operation of the energy source is more expensive, circumstances change that require more power than available from the energy source, or any combination thereof. Such storage systems may take particular advantage of a heat storage device that has low thermal losses to the environment over long periods of storage time, such as the devices according to the teachings herein.

[0087] As discussed hereinbefore, a temperature gradient may develop in a heat storage device, such as while storing heat. The heat storage device may be adapted so that heat from specific regions of the heat storage device are withdrawn depending on the temperature needed for a heat consuming device, the temperature in different regions of the heat storage device, or both. For example, a heat consuming device may require a HTF to be heated to demand temperature, T₀, where one or more caged flow path sections has a temperature below T_D and one or more flow path sections has a temperature greater than T_D. Here, the tank of the device may have a sufficient number of openings and/or may include valves, so that the flow can be controlled to exclude one or more of the caged flow path sections having a temperature below T_D. As another example, all of the TESM in a heat storage device may have a temperature greater than T_D, and it may be desirable to also use the device to also provide heat at the same or a later time for at least a second heat consuming device that requires heat transfer fluid to be heated to a higher temperature, To₂ (i.e., T_D2 > T_D). Here, the HTF for the device that requires a temperature of T_D may advantageously be heated by flowing through a caged flow path section having a relatively low temperature, so that heat in a different caged flow path section having a relatively high temperature can be reserved for heating the second device. Any valves for directing the flow to one or more specific caged flow path sections may be within the tank, outside the tank, or both.

[0088] By way of example, the heat storage system may employ solar radiation as an energy source. During times when i) there is a generally high flux of solar radiation; ii) at least some of the flux of the solar radiation is not needed, or both, some or all of the solar radiation may be converted to heat and stored in a heat storage device for later use. Some of the stored heat may be used at a later time when there is a higher demand for the thermal energy than the available solar radiation flux can provide. For example, some or all of the heat may be stored for heating water at times when it is desirable to use hot water, some or all of the heat may be stored for heating the air in a building at a time when it is desired to increase or maintain the temperature of such airspace, or both. As another example, solar radiation may be employed for generation of electricity, such as by heating a liquid above its boiling point using concentrated solar radiation, for powering a turbine. At certain times, such as when the demand for the electricity is lower than the available power of the solar collector, some or all of the heat from the solar collector may be stored in a heat storage device for later use. Here, the stored heat may be used to generate electricity at a later time when there is greater demand, or the stored heat may be used for a different purpose, such as heating water and or the air space of a building.

[0089] It will be appreciated that the length of time required for storage of the heat will depend on many factors. For example it may be desirable to solar radiation during one day for later use at night, for later use on a cloudy day, for later use when there is a high demand for electricity, for later use when there is a demand for hot water, for later use when a building is occupied, for later use when solar radiation is reduced due to seasonal changes, for later use when ambient temperatures are reduced due to daily, weekly, monthly or seasonal variation, or any combination thereof. As such, a heat storage device and/or a heat storage system according to the teachings herein may be used in a process that includes a step of storing heat for about 8 hours or more, about 3 days or more, about 10 days or more, about 30 days or more, or even about 100 days or more. For example, the heat storage device may be gradually heated during a hot season with reduced or minimal heat losses so that the stored heat may be discharged and utilized on demand during a cold season.

[0090] In one aspect of the invention, the heat storage device is used in a heat storage system that stores heat from a hot season for use in a cold season. Such a system may employ a generally small solar collector, as heat may be collected and stored during a substantial portion of, or even during the entirety of the hot season. In contrast, the solar collector for storing radiation during the daytime in the cold season for later use that same night will typically be much larger due to the reduced solar flux and the shorter number of daylight hours. As a solar collector, and particularly a high-temperature solar collector (such as solar collector that include a solar concentrator), is often an expensive component of a heat storage system, a smaller solar collector may result in substantial cost savings. By using a small solar collector, coupled with the heat storage device according to the teachings herein that require reduced, minimum, or even insulation, it is possible to produce more economical solar heating systems capable of storing heat during a low demand season for later used during a higher demand season.

[0091] As another example, a heat storage system may be employed in a generally warm climate using a PCM having a low liquidus temperature (e.g., about 20 °C or less, about 15 °C or less, or about 10 °C or less) so that heat is removed from the heat storage device during the cold season via a heat exchanger between the HTF and the environment (e.g. cold city water and/or cold ambient air). The heat storage device may then be used during the hot season to cool a building. As such, a heat storage system may be employed for refrigeration purposes.

[0092] A building may employ a plurality of heat storage systems including a first heat storage system that provides heating using a PCM having a liquidus temperature of about 30 °C or more, and a second heat storage system that provides refrigeration using a PCM having a liquidus temperature of about 20 °C or less. One or both of the systems preferably includes a heat storage device according to the teachings herein.

[0093] Preferred heat storage systems have a heat storage device that is sufficiently large so that it can provide the on-demand heat requirements (e.g., for heating a building, heating water or both) for a long continuous period. For example the heat storage device preferably stores a sufficient amount of heat to meet the on-demand requirements for about 30 days or more, about 60 days or more, or about 90 days or more.

[0094] A heat storage system that employs a solar collector may include a solar concentrator so that the heat may be stored at generally high temperatures. Any solar concentrator capable of increasing the flux of radiation may be used. Preferred solar concentrators increase the flux of solar radiation by about 100% or more. Any solar concentrator known to one of ordinary skill in the art of concentrating solar light may be employed. Solar concentrators that may be employed include those taught by US Patent Numbers 4.022,184, 4,286,579, and 6,131 ,565, which are incorporated herein by reference in their entirety. Examples of solar concentrators include parabolic reflectors and Fresnel reflectors.

[0095] A solar concentrator may be particularly advantageous in a heat storage system that employs a PCM having a high liquidus temperature, such as a liquidus temperature of about 80 °C or more, about 100 °C or more, about 120 °C or more, about 140 °C or more, about 160 °C or more, about 180 °C or more, or about 200 ^eC or more. When using a solar collector and/or a PCM having a high liquidus temperature, it may be necessary to use a HTF that is chemically stable, has fairly low vapor pressure at the elevated temperature of the system, or both. For example, the HTF may include or consist essentially of one or more synthetic organic fluids, one or more silicon fluids, or both. Examples of commercially available heat transfer fluids that are stable at a liquidus temperature of these PCMs include DOWTHERM™ brand synthetic organic fluids (commercially available from THE DOW CHEMICAL COMPANY), SYLTHERM™ (trademarked by DOW CORNING CORPORATION and distributed by THE DOW CHEMICAL COMPANY), and THERMINOL® brand heat transfer fluid (commercially available from SOLUTIA INC.).

[0096] The heat storage system may include one or more charging loops for charging the heat storage device. The charging loop may be in thermal communication with a component that supplies heat for the heat storage device. The charging loop may include one or more cold line that supplies relatively cold HTF from the heat storage device to the heat source, such as a solar collector, or to a component that is in thermal communication with the heat source. The charging loop may include one or more hot lines that supplies a relatively hot transfer fluid (e.g., hotter than the HTF in the cold line), from the heat source or from the component in thermal communication with the heat source to the heat storage device. The charging loop may include one or more valves capable of controlling the flow through the charging loop. The charging loop may include a pump for pumping the HTF. If employed, the pump is preferably located on the cold line side of the charging loop so that the pump is not exposed to temperatures that may degrade its seals or cause them to leak.

[0097] The heat storage system may include one or more discharging loops for discharging the heat storage device. The discharging loop may be in thermal communication with a heat consuming device that requires on-demand heat. The discharging loop may include one or more cold line that supplies relatively cold HTF from the heat consuming device or from an intermediate component that is in thermal communication with the component heat consuming device to the heat storage device. The discharging loop may include one or more hot lines that supplies a relatively hot transfer fluid (e.g., hotter than the HTF in the cold line), to the heat storage device from the heat consuming device or from the intermediate component in thermal communication with the heat consuming device. The discharging loop may include one or more valves capable of controlling the flow of HTF through the discharging loop. The discharging loop may include a pump for pumping the HTF. If employed, the pump is preferably located on the cold line side of the discharging loop so that the pump is not exposed to temperatures that may degrade its seals or cause them to leak.

[0098] A discharging loop and a charging loop may flow through the heat storage device using the same flow path. Alternatively, the heat storage device may employ different paths for flowing charging loop and the discharging loop. In the later case, it may be advantageous to employ different HTFs for the charging loop and the discharging loops.

[0099] The system may include a plurality of charging loops or a plurality of discharging loops.

For example, a system may include different discharging loops for providing heat to different heat consuming devices that require HTF at different temperatures.

[00100] A heat storage system may include one or more temperature sensors for measuring the temperature of a hot "line, a cold line, a heat storage device, a heat source, an on-demand component, or any combination thereof. The heat storage system may include a sufficient number of temperature systems so that a controller can determine whether or not to circulate HTF in a charging loop, in a discharging loop, or both. The controller may operate by controlling one or more valves, by controlling one or more pumps, or both. The controller may include separate control devices for controlling the charging loop and the discharging loop. Preferably, the system includes separate pumps for the charging loop and the discharging loop so that the charging loop and the discharging loop may be operated independently. For example a first pump in the charging loop may operate when there is solar thermal energy available for storage and a second pump in the discharging loop may operate when the heat consuming device demands heat. The pumping rate of the pump in the charging loop may be controlled by a controller based on one or more sensors that measure a temperature of the solar collector and/or based on one or more sensor that measure the amount of available solar radiation. The pumping rate of the pump in the discharge loop may be controlled by a controller that senses the needs of the heat- consuming device.

[00101] To reduce or minimize heat losses, one or any of the devices and components in the heat storage system may be thermally insulated. Preferably, at least the hot lines are thermally insulated so that heat loss to the environment are reduced or minimized.

[00102] With reference to FIG. 12, a heat storage system 120 may include a charging loop 124. The charging loop 124 may include a pump 128 for circulating a HTF 8 between a heat storage device 80 and a solar collector 126. The charging loop may include a cold line 130 for flowing relatively cold HTF 8 from the heat storage device 80 to the solar collector 26 and a hot line 32 for flowing relatively hot HTF 8 from the solar collector 126 to the heat storage device 80. The heat storage preferably includes three or more caged flow path sections 20. The heat storage system 120 may include a discharging loop 134. The discharging loop 134 may include including a pump 138 for circulating a HTF 8 between a heat storage device 80 and a heat consuming device 136. The discharging loop 134 may include a cold line 140 for flowing relatively cold HTF 8 from the heat consuming device 136 to the heat storage device 80. The discharging loop 134 may include a hot line 142 for flowing relatively hot HTF 8 from the heat storage device 80 to the heat consuming device 136. The heat storage system preferably employs insulation 82 for reducing or minimizing heat losses to the environment. Insulation may be used on any of the components and devices. Preferably insulation is used on one or more the hot lines 132, 142 and/or the heat storage device 80.

[00103] A heat storage system may include one or more heat exchangers. For example, a heat exchanger may be employed between a solar collector and a heat storage device so that heat is first transferred from the solar collector to the heat exchanger and then to the heat storage device. As another example, a heat exchanger may be employed between a heat storage device and a heat consuming device so that during discharging heat from the heat storage device, heat is first transferred from the heat storage device to the heat exchanger and then to the heat consuming device. A single heat exchanger may be employed for more than one heat consuming devices. A single heat exchanger may be employed for both charging and discharging a heat storage device.

[00104] Instead of heating a single heat consuming device, a single heat storage device may advantageously be used for providing heat to two or more heat consuming devices. For example a heat storage device may provide heat to two or more heat consuming devices wherein at least one of the heat consuming devices is an air heater for a building, a hot water heater for hot water plumbing, a device for producing steam, a device employed in an industrial process, such as a chemical reaction, a device that includes a heat engine such as a Rankine or Stirling cycle, or a heat-consuming refrigeration device based on an absorption cycle or adsorption cycle.

[00105] The heat storage system may also include one or more by-pass valves so that the HTF can circulate between the heat source and the heat consuming device without flowing through the heat storage device.

[00106] While the present invention may be susceptible to various modifications and alternative forms, the exemplary embodiments discussed above have been shown by way of example. However, it should again be understood that the invention is not intended to be limited to the particular embodiments disclosed herein. Indeed, the present techniques of the invention are to cover ail modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the following appended claims.

[00107] Any numerical values recited herein include all values from the lower value to the upper value in increments of one unit provided that there is a separation of at least 2 units between any lower value and any higher value. As an example, if it is stated that the amount of a component or a value of a process variable such as, for example, temperature, pressure, time and the like is, for example, from 1 to 90. preferably from 20 to 80, more preferably from 30 to 70, it is intended that values such as 15 to 85, 22 to 68, 43 to 51, 30 to 32 etc. are expressly enumerated in this specification. For values which are less than one, one unil 5 considered to be 0.0001 , 0.001 , 0.01 or 0.1 as appropriate. These are only examples of what is specifically intended and all possible combinations of numerical values between the lowest value and the highest value enumerated are to be considered to be expressly stated in this application in a similar manner.

[00108] Unless otherwise stated, all ranges include both endpoints and all numbers between the endpoints. The use of "about" or "approximately" in connection with a range applies to both ends of the range. Thus, "about 20 to 30" is intended to cover "about 20 to about 30", inclusive of at least the specified endpoints.

[00109] The disclosures of all articles and references, including patent applications and publications, are incorporated by reference for all purposes. The term "consisting essentially of to describe a combination shall include the elements, ingredients, components or steps identified, and such other elements ingredients, components or steps that do not materially affect the basic and novel characteristics of the combination. The use of the terms "comprising" or "including" to describe combinations of elements, ingredients, components or steps herein also contemplates embodiments that consist essentially of the elements, ingredients, components or steps. By use of the term "may" herein, it is intended that any described attributes that "may" be included are optional.

[00110] Plural elements, ingredients, components or steps can be provided by a single integrated element, ingredient, component or step. Alternatively, a single integrated element, ingredient, component or step might be divided into separate plural elements, ingredients, components or steps. The disclosure of "a" or "one" to describe an element, ingredient, component or step is not intended to foreclose additional elements, ingredients, components or steps.

Claims

CLAIMS What is claimed is

1. A heat storage device comprising:

a tank having a cavity;

a thermal energy storage material in the cavity of the tank;

two or more openings in the tank so that a heat transfer fluid can enter and exit the cavity of the tank through different openings; and

one or more flow path structures within the cavity that define a predetermined flow path through the cavity of the tank, wherein the flow path structures are configured to prevent the thermal energy storage material from directly contacting the heat transfer fluid, wherein the flow path structures are in fluid communication with two or more openings of the tank;

wherein the flow path has two or more caged flow path sections and includes i) a first caged flow path section that defines a 2-dimensional boundary surface , wherein the cavity has a space inside the boundary surface of the first caged flow path section and a space outside the boundary surface of the first caged flow path section;

ii) a second caged flow path section that is smaller than the first caged flow path and is located within the space inside the boundary surface of the first caged flow path section; and

iii) a gradient flow path section that provides a fluid connection between the first and second caged flow path sections so that a heat transfer fluid can flow serially through both the first and second flow path sections.

. The heat storage device of claim 1 , wherein

a) one of the caged flow path section generally follows an isotherm surface,

b) the second caged flow path section generally follows an isotherm surface, or

c) two or more caged flow path sections generally follows different isotherm surfaces.

. The heat storage device of claim 1 or 2, wherein the one or more flow path structures includes a pipe or two or more spaced apart walls, or both, wherein the two or more spaced apart walls.

. The heat storage device of any of claims 1 through 3, wherein the two or more spaced apart walls includes two spaced apart hollow objects each having generally solid outer surfaces, wherein one object is interior to the other object, and wherein the objects are generally ellipsoidal, generally cylindrical, or generally cuboid.

5. The heat storage device of any of claims 1 through 4, wherein the second caged flow path section is defined by a flow path structure that includes a plurality of windings of pipe, wherein the windings includes two adjacent windings having different circumferences.

6. A heat storage device comprising:

i) a tank having a cavity, and two or more openings for flowing a heat transfer fluid through the cavity;

ii) one or more pipes in the cavity of the tank and in fluid communication with two or more openings so that a heat transfer fluid can flow through the one or more pipes, wherein the one or more pipes includes two or more isothermal portions of pipe including:

a first isothermal portion of pipe having a plurality of windings including adjacent windings having different circumferences, and having a length direction that generally follows a first isothermal contour surface; and

a second isothermal portion of pipe having a plurality of windings including adjacent windings having different circumferences, and having a length direction that generally follows a second isothermal contour surface interior to the first isothermal contour surface;

so that heat flow along the length of the isothermal portions of pipe is reduced or minimized; and

iii) a thermal energy storage material in the cavity and exterior to the pipe, wherein the thermal energy storage material is in thermal communication with the pipe;

wherein the one or more pipes have sufficiently high thermal conductivity and surface area so that heat can be transferred between the thermal energy storage material and a heat transfer fluid flowing through the pipe.

. The device of any of claims 1 through 6, wherein the device includes one or more support components suitable for positioning a flow path structure, which may be a pipe section, in a predetermined location and / or arrangement so that the heat transfer fluid can flow along a generally isothermal contour surface.

. The device of any of claims 1 through 7, wherein an isothermal portion of pipe includes adjacent windings of pipe having different pitch and different circumference.

. The device of any of claims 1 through 8, wherein two adjacent caged flow path sections are in fluid communication by a single connection.

0. The device of any of claims 1 through 8, wherein the device includes a surface extender component in thermal communication with a pipe and the thermal energy storage material, wherein the surface extender component has an elongated shape including a length that is greater than its thickness, and has a thermal conductivity greater than the thermal conductivity of the thermal energy storage material, so that heat can be more rapidly transferred between the pipe and at least some of the thermal energy storage material that is positioned away from the pipe.

11. The device of any of claims 1 through 10, wherein the thermal energy storage material includes a phase change material having a solid to liquid phase transition at a temperature greater than about 50 °C, and wherein the tank is thermally insulated.

2. The device of any of claims 1 through 11 , wherein the thermal energy storage material is not contained in a plurality of individually sealed spaces.

13. The device of any of claims 1 through 12, wherein

the flow path includes two or more caged flow path sections; and

i) the flow path includes a gradient flow path section that flows in a non-isothermal direction and provides a fluid connection between two caged flow path sections,

ii) the flow path includes a gradient flow path section that flows in a non-isothermal direction and provides a fluid connection between a first caged flow path section and a first opening of the tank;

iii) the flow path includes a gradient flow path section that flows in a non-isothermal direction and provides a fluid connection between a second caged flow path section and a second opening of the tank; or

iv) any combination of i, ii, and iii.

4. The device of any of claims 1 through 3, wherein a wall defining a gradient flow path section includes a material having a thermal conductivity less than the thermal conductivity of a material of a wall that defines a caged flow path section.

5. A system including a device of any of claims 1 through 14, wherein the system includes a heat source; wherein the heat storage device and the heat source are in thermal communication at least during one or more instances when the heat source has a temperature greater than the temperature of the thermal energy storage material in the heat storage device.

6. The use of a heat storage device of any of claims 1 through 14 for heating a building, heating a water tank, heating an industrial process, or any combination thereof.

7. A process comprising:

a step of flowing a heat transfer through a flow path in a cavity of a tank;

wherein the step of flowing through a flow path includes:

serially flowing through a plurality of caged flow path sections including serially flowing through

i) a first caged flow path section having an adjacent caged flow path section;

ii) a gradient flow path section that provides a fluid connection between the two adjacent caged flow path sections; and

iii) the adjacent caged flow path section;

wherein the two adjacent caged flow path sections have different sizes and each define a bounded volume, and wherein one of the caged flow paths sections is located within the bounded volume of the other.

18. A process comprising:

a step of charging a heat storage device of any of claims 1 through 14 including

i) flowing a heat transfer fluid into an opening of a tank and through the flow path in the cavity of the tank, wherein the thermal energy storage material includes a phase change material, and wherein the heat transfer fluid has an initial temperature that is greater than a temperature of the phase change material contained in the cavity; ii) contacting the heat transfer fluid to a wall along one or more caged flow path sections; iii) transferring heat from the heat transfer fluid to the wall;

iv) conducting the heat through the wall to a phase change material;

v) melting at least a portion of the phase change material; and

vi) flowing the heat transfer fluid out of the cavity and through a second opening wherein the heat transfer fluid has a temperature low than its initial temperature. 9. The process of claim 17 or 18, wherein the process comprises:

a step of circulating a heat transfer fluid through a charging loop so that heat form a heat source is stored in the heat storage device;

a step of circulating a heat transfer fluid through a discharging loop so that heat stored in a heat storage device is used to heat one or more objects;

or both.

0. The process of any of claims 17 through 19, wherein the process comprises a step of: i) concentrating sunlight using a solar concentrator;

ii) heating a heat transfer fluid using source heat from the concentrated sunlight so that the heat transfer fluid has a temperature of about 200 °C or more; and

iii) circulating the heat transfer fluid through a charging loop so that the thermal energy storage material in the heat storage device is heated to a temperature of about 200 °C, or more.